1. Field of the Invention
The present invention generally relates to a driving method of liquid crystal display (LCD). More particularly, the present invention relates to a driving method of LCD having high aperture ratio and high stability of displaying gray scale picture.
2. Description of Related Art
In general, conventional liquid crystal display (LCD) may be classified into passive LCD and active LCD according to the driving method thereof. For example, the LCD provided for conventional mobile phone is generally a passive LCD. The conventional passive LCD has the disadvantage of low image,residual shadowing, low contrast and low response speed due to the coupling of the capacitor generated in the passive LCD. In addition, the structure of the passive LCD is more complex than that of the active LCD since the passive LCD is driven by multiplex driving process. Therefore, for a passive LCD, to achieve high resolution, high definition, and full-color is difficult. However, since the cost for manufacturing a passive LCD is low, the passive LCD is generally provided for low stage display device.
Alternatively, an active LCD is generally provided for the thin film transistor (TFT) LCD display device of notebook or monitor. In general, the disadvantage of the passive LCD described above is improved in the active LCD, therefore the image quality and the resolution of the active LCD is much better than that of the passive LCD. An important improvement made to the active LCD is that TFT is provided as a switching device for controlling the rotation and the direction of the liquid crystal molecule. FIG. 1 schematically illustrates a driving circuit of a conventional thin film transistor (TFT). Referring to FIG. 1, the driving circuit includes a data line 103, a gate line 105, a thin film transistor (TFT) 107, a liquid crystal capacitor 109 and a storage capacitor 111. First of all, application of a voltage to each pixel of the liquid crystal molecule in the LCD will be described in the following. In an active LCD, each pixel has a TFT 107. The gate of the TFT 107 is connected to a horizontal gate line such as the gate line 105 shown in FIG. 1. The source of the TFT 107 is connected to a vertical-data line such as the data line 103 shown in FIG. 1. The drain is connected to a pixel electrode. It is noted that, the source and the drain can be applied with the voltages of the data line and the pixel electrode respectively. When the active LCD is operated, the voltages described above do not remain at a constant level but is repeatedly changed in a range of voltage acceptable for the liquid crystal molecule.
Hereinafter, the operation method of the LCD will be described. First of all, a gate line 105 is activated to turn on all the thin film transistors connected to the gate line 105 such as the TFT 107. Next, a corresponding data signal for charging the pixel electrode to an applicable voltage is provided via the data line 103. Thereafter,the TFT 107 is turned off until the gate line 105 is activated subsequently. In the meanwhile, the charge is stored in the liquid crystal capacitor 109. Then, the next gate line is activated for writing the corresponding data signal to the thin film transistors connected to the next gate line. After all the data of the whole frame is written via all the gate lines of the frame gradually. The foremost gate line is activated again to write the signal. Therefore, the cross talk between each pixel is reduced by the simple driving method described above. It is noted that the imaging quality of the LCD is mainly dependent on the electrical characteristics of the TFT such as the turn-off current, the driving current, the parasitic capacitance and the switching speed of the TFT.
The storage capacitor 111 is provided for storing the charge. However,the storage capacitor can also reduce the coupling effect of the voltages applied to the liquid crystal molecule, i.e., the differential voltages between each pixel electrode and the common pixel electrode. When the TFT is turned off, the pixel electrode is not applied with any voltage, i.e., floated. However, at this moment, any variation of voltage around the pixel electrode will be coupled to the pixel electrode via the parasitic capacitance, and thus the voltage level of the pixel electrode is changed. Therefore, the voltage applied to the liquid crystal molecule is influenced. It is noted that an increase in the storage capacitor can reduce the coupling effect between the voltages. However, since at least one pixel electrode of the two pixel electrodes of the storage capacitor is composed of non-transparent metal, the larger the capacitance of the storage capacitor, larger the area of the storage capacitor is. Therefore, the transparent portion of each pixel is reduced, and thus the total light emitting efficiency of the LCD is reduced. Therefore, in order to enhance the aperture ratio of the LCD, the source/drain region of the LCD is generally manufactured by full self alignment process to reduce the parasitic capacitance and the size of the storage capacitor.
Hereinafter, the driving device will be described. FIG. 2 schematically illustrates a conventional driving device of a TFT array using 3N*1driving method. Referring to FIG. 2, the driving device includes pixel G222 for displaying green color, pixel B225 for displaying blue color, pixel R227 for displaying red color, and coupling capacitance 201, 203, 205, 207, 209, 211, 213, 215, 217 and 219 of each pixel. In the 3N*1driving method, the polarity distribution of the voltages of the pixel electrodes of the gate lines in the same horizontal line is +++−−−+++−−−+++−−− . . . In other words, the polarity of the voltages of the pixel electrodes are changed every three pixels due to the three primary colors, green, blue and red. It is noted that, in the 3N*1driving method described above, although the possibility of generation of the traverse electric field is reduced, however, the brightness of the frame is not uniform due to the following reasons.
Referring to FIG. 2, for the pixel G222, since the polarity of voltages of two adjacent data lines are the same, there is no traverse electric field between the two data lines, and thus the aperture ratio is high. However, since the coupling capacitance 205 and 207 may be coupled and an adding effect of the two coupling capacitance is generated, the cross talk therebetween is enhanced. The effect described above may also be generated in the pixel R227.
Referring to FIG. 2, for the pixel B225, since the polarity of voltages of the two adjacent data lines are different, an subtracting effect of the coupling capacitance 209 and 211 is generated corresponding to the pixel B between the two adjacent data lines, and thus the cross talk there-between is reduced.
In summary, in the three pixel of the same frame, the brightness of the pixel B225 is not the same as that of the pixels G222 and R222, therefore the image displayed by the panel is not uniform. Accordingly, although the 3N*1driving method can increase the aperture ratio, however, the displayed image of the frame is not uniform.